1. Field of the Invention
The present invention relates generally to an interpolation signal producing circuit, and more particularly is directed to a relatively simple interpolation signal producing circuit which can produce various kinds of interpolation signals, for example, for doubling scanning lines, or for compensating for dropped-out pixels and the like.
2. Description of the Prior Art
In a recently proposed television receiver, a reproduced picture of high definition is to be obtained by increasing the number of pixels (picture elements) in each field of a received video signal. According to this proposal, resolution in the vertical direction of the reproduced picture is improved by a so-called double-scanning system in which horizontal scanning lines in one field of the video signal are doubled and non-interlace scanning is carried out at each field. On the other hand, resolution in the horizontal direction of the reproduced picture is improved by a so-called sub-sampling system in which the number of pixels within one horizontal scanning line is doubled by inserting interpolation signals therebetween.
A case in which horizontal scanning lines are doubled in order to effect the double-scanning will now be explained with reference to FIGS. 1A-1C. An interpolated horizontal scanning line is shown by a dashed line in FIG. 1B to be provided between two adjacent horizontal lines of an (n)th field, and a predetermined pixel x.sub.1 is formed on this interpolated horizontal scanning line either by an interfield interpolation method or by an intrafield interpolation method.
In the intrafield interpolation method, the amplitude of the interpolation pixel x.sub.1 is determined from the amplitudes of adjacent pixels a.sub.1 and b.sub.1 of upper and lower horizontal scanning lines, as x.sub.1 =(a.sub.1 +b.sub.1)/2.
In the interfield interpolation method, the amplitude of the interpolation pixel x.sub.1 is determined from a pixel c.sub.1 (FIG. 1A) located at the same position as the interpolation pixel x.sub.1, but in an (n-1)th field, that is, one field before the (n)th field.
Advantages and disadvantages of the interfield interpolation method and the intrafield interpolation method will now be described and, in connection therewith, it will be assumed that a level "1" indicates white and a level "0" indicates black. Further, it will be assumed that a still picture to be reproduced is as shown in FIG. 2, and wherein a rectangular white image W appears against a black background B. In such case, an interpolation pixel x.sub.1, formed on the boundary portion between the white image W and the black background B, is represented as x.sub.1 =(a.sub.1 +b.sub.1)/2=(1+0)/2=0.5 according to the intrafield interpolation method. On the other hand, the same interpolation pixel is represented as x.sub.1 =c.sub.1 =1 (where pixel c.sub.1 is located at the same position in the preceding field and is assumed to be white) according to the interfield interpolation method. Thus, the interfield interpolation method can correctly form the interpolation pixel, whereas the intrafield interpolation method provides a color level which is a mixture of white and black at the boundary portion and thereby results in an indistinct boundary between the white image and the black background.
Further, in the case of a real moving picture in which a rectangular white image W is moved relative to a black background B, as shown in FIG. 3 from a position shown by a dashed-line in an (n-1)th field to the position shown in full lines in the next or (n)th field, so that the white images W of the (n-1)th field and of the (n)th field do not overlap each other, then an interpolation signal x.sub.1 ' formed within the white image W is represented as x.sub.1 '=(a.sub.1 '+b.sub.1 ')/2=1 according to the intrafield interpolation method. In the same case, the interpolation signal is represented as x.sub.1 '=c.sub.1 '=0 (c.sub.1 ' is a pixel in the same position as the interpolation pixel x.sub.1 ' but in the preceding field) according to the interfield interpolation method. As a result, for the situation shown in FIG. 3, the intrafield interpolation method can correctly form the interpolation pixel x.sub.1 ' whereas the interfield interpolation method causes a black pixel to occur in the white image W, thus forming an erroneous interpolation signal.
In order to avoid the above-described defects inherent in the prior-art interpolation, a combination of the interfield interpolation method and the intrafield interpolation method has been proposed. In accordance with such proposal, motion in a picture is initially detected, and an interpolation pixel signal x.sub.1 is formed with respect to a still picture portion by the interfield interpolation method, whereas an interpolation pixel signal x.sub.1 is formed with respect to a moving picture portion by the intrafield interpolation method. A circuit employing the known combination of the interfield and intrafield interpolation methods is shown in FIG. 4, in which an input terminal 1 receives a signal at a point a on a predetermined horizontal scanning line, hereinafter referred to as the (m)th line, and an input terminal 2 receives a signal at a point b.sub.1 on a predetermined horizontal scanning line hereinafter referred to as (m+1)th line. The signals a.sub.1 and b.sub.1 applied to the input terminals 1 and 2 are supplied to an adder 3 and the resultant signal is supplied to an attenuator 4 in which its signal level is attenuated by one-half. The attenuated signal is supplied to one end of the resistance element of a variable resistor 5. A signal at a point c on the (m)th line of the preceding or (n-1)th field is applied to an input terminal 6, and is fed therefrom to the other end of the resistance element of the variable resistor 5 whose slide contact is connected to an output terminal 7 from which an interpolation signal is derived.
The slide contact of the variable resistor 5 is changed in position under the control of a movement detection circuit 8. When movement in an input video signal is detected by the circuit 8, the slide contact of the variable resistor 5 is moved to one end of the resistance element, so that an interpolation signal provided by the intrafield interpolation method is supplied to the output terminal 7. When the input video signal is detected to be a still picture by the movement detection circuit 8, the slide contact of the variable resistor 5 is moved to the other end of the resistance element for thereby supplying to the terminal 7 an interpolation signal provided by the interfield interpolation method. In this fashion, the intrafield interpolation method and the interfield interpolation method are selectively used on the basis of whether or not movement in the input video signal is detected by the movement detection circuit 8.
It is also known to employ sub-sampling for doubling the number of pixels within each horizonal scanning line. For example, as shown in FIG. 5C, an interpolation pixel x.sub.2 may be provided between pixels b.sub.2 and c.sub.2 on a certain horizontal scanning line of an (n)th field by either a intrafield interpolation method or an interframe interpolation method. In the case of the intrafield interpolation method, the interpolation pixel x.sub.2 is formed from adjacent pixels a.sub.2 and d.sub.2 on the adjacent upper and lower horizontal scanning lines and the adjacent pixels b.sub.2 and c.sub.2 on the same horizontal scanning line so that x.sub.2 =(a.sub.2 +b.sub.2 +c.sub.2 +d.sub.2)/4 is established. In the case of the interframe interpolation method, the interpolation pixel x.sub.2 is formed from a pixel e.sub.2 located at the same position as that of the interpolation pixel x.sub.2, but in the (n-2)nd field (FIG. 5A) which is one frame before the (n)th field, and x.sub.2 =e.sub.2 is established.
The results of these two methods will be compared for the case of a still picture formed of a black background B with a rectangular white image W located within the background B, as shown in FIG. 6. An interpolation pixel x.sub.2 formed at the boundary portion of the white image W by the intraframe interpolation method (assuming that only the pixel d.sub.2 on the lower horizontal line is located within the black background B) is expressed as x.sub.2 =(a.sub.2 +b.sub.2 +c.sub.2 +d.sub.2)/4=(1+1+1+0)/4=0.75. On the other hand, interpolation pixel x.sub.2, formed by the interframe interpolation method, is expressed as x.sub.2 =e.sub.2 =1. Therefore, it will be seen that, in the case of a still picture, the interpolation pixel is correctly formed by the interframe interpolation method, whereas the intrafield interpolation method causes a color level corresponding to a mixture of white and black to occur at the boundary portion which is thereby made indistinct.
In the case of a real moving picture in which a rectangular white image W is moved within a black background B, as shown in FIG. 7, and assuming that the white portion W of the preceding frame, or (n-2)nd field, is located as shown by a broken line in FIG. 7 and does not overlap the range of the white image W of the present or (n)th field, if an interpolation pixel x.sub.2 ' is to be formed at a boundary portion of the white image W where only the adjacent pixel c.sub.2 ' on the right is located within the black background B and the other pixels a.sub.2 ', b.sub.2 ' and d.sub.2 ' are located within the white image W, then, in accordance with the intrafield interpolation method, the interpolation pixel x.sub.2 ' is expressed as x.sub.2 '=(a.sub.2 '+b.sub.2 '+c.sub.2 '+d.sub.2 ')/4=(1+1+0+1)/4=0.75. On the other hand, in accordance with the interframe interpolation method, the interpolation pixel x.sub.2 ' is expressed as x.sub.2 '=e.sub.2 '=0 (pixel e.sub.2 ' corresponds to the interpolation pixel x.sub.2 ' of the preceding frame). Therefore, it is apparent that the correct interpolation pixel is formed by the intrafield interpolation method, whereas, the interframe interpolation method provides a black pixel within the white image W, that is, an erroneous interpolation signal is formed.
In order to avoid the above-described defects inherent in the prior art, it has been proposed that, interpolation is carried out, motion of the picture be detected, and interpolation signal x.sub.2 be formed by the interframe interpolation method for a still picture portion, whereas, interpolation signal x.sub.2 ' is formed by the intrafield interpolations method for the real moving picture portion.
More specifically, in a circuit shown in FIG. 8 for combining the intrafield and interframe interpolation methods, a signal at a point b.sub.2 on a predetermined horizontal scanning line, which is hereinafter referred to as (m)th line in an (n)th field, is applied to an input terminal 11, and a signal at a point c.sub.2 also on the (m)th line of the (n)th field is supplied to an input terminal 12. A signal at a point a.sub.2 on an (m-1)th line is supplied to an input terminal 13, and a signal at a point d.sub.2 on an (m+1)th line is supplied to an input terminal 14. These signals applied to the input terminals 11, 12, 13 and 14 are supplied to an adder 15, and the resultant added signal is supplied to an attenuator 16, in which its signal level is attenuated by one-fourth. This attenuated or average signal is supplied to one end of the resistance element of a variable resistor 17. Further, a signal at the point e.sub.2 of the (m)th line of the (n-2)nd field, that is, one frame before the (n)th field, is applied to an input terminal 18, and is supplied therefrom to the other end of the resistance element of the variable resistor 17. A slide contact of the variable resistor 17 is connected to an output terminal 19 from which an interpolation signal may be derived. The slide contact of the variable resistor 17 is changed in position under the control of a movement detection circuit 20. Thus, when a movement is detected in an input video signal by the circuit 20, the slide contact of the variable resistor 17 is moved to one end of the resistance element for supplying to the output terminal 19 an interpolation signal produced by the intrafield interpolation method. Conversely, when the detecting circuit 20 indicates that the input video signal is a still picture, circuit 20 causes the slide contact of the variable resistor 17 to be moved to the other end of the resistance element for supplying an interpolation signal produced by the interframe interpolation method to the output terminal 19. In this fashion, the intrafield interpolation method and the interframe interpolation method are selectively used on the basis of whether or not motion in the picture is detected by the circuit 20.
It is apparent from the above that satisfactory interpolation cannot be achieved in either double-scanning or sub-sampling without detecting the motion of a picture and, in practice, it is very difficult to carry out perfect movement detection for the reasons described more fully below with reference to FIGS. 9A to 9D.
FIGS. 9A to 9D illustrate the assumed positions of two pulse signals P.sub.1 and P.sub.2 representing white portions on a predetermined horizontal line, and which are sequentially moved at the successive fields (n-2), (n-1), (n), and (n+1). When the pixel position of the pulse signal P.sub.1 of the (n)th field (FIG. 9C) coincides with the pixel position of the pulse signal P.sub.2 of the (n-2)nd field, that is, one frame before (FIG. 9A), if the movement detection is carried out on the basis of the pixel position of the pulse signal P.sub.1 of the (n)th field, then the pixel is detected as a still picture portion having no motion. In other words, in the illustrated states of FIGS. 9A and 9C, or of FIGS. 9B and 9D, the pulse signal P.sub.1 and the pulse signal P.sub.2 are at the same horizontal positions along the respective line so that the difference between the frames at this pixel position cannot be detected from the signal level or the like. As a result, an interpolation signal for a still picture may be produced as an interpolation signal for a real moving picture, so that an erroneous interpolation signal is produced and the image quality is thereby deteriorated. This is the reason for the frequent observation that interpolation-processing, intended to improve the image quality, causes instead a decrease in the vertical and/or horizontal resolution.
Further, the above-described movement detection circuit is complicated and expensive, and cannot be readily utilized with a television receiver.
Furthermore, when a drop-out occurs in a reproduced image signal, or when the level of a reproduced image signal is lowered in a video apparatus, such as, a video tape reproducing apparatus or the like, the signal portion in which the drop-out occurs is replaced with a signal, for example, from the preceding horizontal line, under the control of a drop-out detecting circuit, for preventing noise from occurring in the reproduced picture.
However, it is not always optimal to replace a drop-out signal with a signal from the preceding horizontal line. For example, in the case of a still picture, it may be better to replace the drop-out signal with a signal from an adjacent horizontal line of the preceding field. Such processing requires the use of a movement detection circuit similar to that used for the double-scanning and sub-sampling, and the movement detection circuit makes the circuit arrangement complicated, as described above. Thus, in the prior art, the movement detection is usually not carried out, and a signal of the preceding horizontal line is usually utilized as an interpolation signal.